PSYC55 - Cognitive Neuroscience Ch. 2

PSYC55 - CHAPTER 2
 dementia praecox is the same name for schizophrenia
o discovered by Bleuler
o disorder of cognition (sometimes called a thought disorder)
o genetic  has biological component
Cells of the Nervous System
 Cajal observed that although neurons are close to each other, they are separated by small gaps
 Cajal defined 2 main principles of neurons
o Connectional specificity – cells are separate because cytoplasms are not in contact;
circuits pass information through specific pathways
o Dynamic polarization - some parts of the neuron are specialized for taking info in while
others send it out
 2 main classes of cells in nervous system
o Neurons
o Glial cells
Structure of Neurons
 Distinguished by form, function, location, interconnectivity
 Take in info, make decision and pass it along
 Consists of
o Cell body – metabolic machinery to maintain cell
o Dendrites – receive input from synapse (said to be postsynaptic since they come after
the synapse)
o Axons – said to be presynaptic since it appears before the synapse
 Most neurons are both pre and postsynaptic
 Activity within a neuron involves changes in electrical state while at the synapse the signal
between neurons is usually mediated by chemical transmission
 Dendrites take varied and complex forms
o Can exhibit spines: little knobs attached by small necks to the surface of the dendrites;
synapses are located on these spines
 Axon terminals have specialized intracellular structures that enable communication via the
release of neurotransmitters
 Dendrites and axon are extensions of the cell body; filled with the same cytoplasm
o Continuity of the intracellular space between these neuronal components is necessary
for the electrical signaling that neurons perform
 Four general morphological classifications of neurons
o Unipolar
 Only 1 process extending from the body
 Common in invertebrates
o Bipolar
 Have 2 processes (one axon and one dendrite)
 Participate in sensory processes
 Called prototypical: info comes in one end via the dendrite and leaves through
the other end down the axon  Ex. Cells of the retina
o Pseudounipolar
 Appear to be unipolar but are originally bipolar sensory neurons whose
dendrites and axon have fused
 Ex. Dorsal root ganglia of spinal cord
o Multipolar
 Have one axon but many dendrites
 Participate in motor and sensory processing
 Make up majority of neurons in the brain
 Ex. Spinal motor neurons, cortical sensory neurons
Role of Glial Cells (neuroglial cell)
 More numerous than neurons (1:10)
 Account for more than half of brains volume
 Do not conduct signals but neurons need them to function
 Literally means “nerve glue”
 In the CNS (brain and spinal cord) and PNS (motor and sensory outputs to the brain and spinal
cord)
 CNS has 3 main types of glial cells
o Astrocytes
 Large, round, symmetrical
 Surround neurons and come in close contact with brain’s vasculature
 Make contact with blood vessels at end feet which allow the astrocyte to
transport ions across and create barrier between the tissues of CNS and blood
(blood-brain barrier)
o Microglial cells
 Small, irregularly shaped
 Come into play when tissue is damaged
 Serve a phagocytic role, devouring damaged cells
 Can proliferate whereas neurons cannot
o Oligodendrocytes
 Forms myelin
 In PNS, Schwann cells form myelin
 The myelin is wrapped around the axon until the cytoplasm is squeezed out
leaving just the glial cell membrane
 1 oligodendrocyte in the CNS can form myelin sheath around several axons
whereas Schwann cells can only form myelin only for a single axon in the PNS
 Goal of myelin = to provide electrical insulation around the axon that changes
the way intracellular electrical currents flow
 In myelinated axons, the myelin is interrupted at the nodes of Ranvier
Neuronal Signaling
Overview of Neuronal Communication
 Goal of neuronal processing = take in info, evaluate it, and pass a signal to the other neurons
 Neurons first receive a signal that is either chemical or physical
 Signal initiates changes in membrane of postsynaptic neuron  Current flow is mediated by ionic currents carried by electrically charged ions
 Action potentials can be generated in a spike-triggering zone of the neuron that integrates the
currents from many synaptic outputs
 Signal travels down the axon to its terminals and eventually causes the release of
neurotransmitters at synapse
Properties of the Neuronal Membrane and the Membrane Potential
 Neuronal membrane is a bilayer of lipid molecules that separates intracellular space from
extracellular space
 It does not dissolve in the watery environments inside and outside the neuron
 The membrane has many transmembrane proteins including ion channels and active
transporters or pumps
 Resting membrane potential = the difference in voltage across the neuronal membrane
 Ion channels are formed by transmembrane proteins that create pores
 Thousands of ion channels exist in the neuronal membrane
 Some ion channels are passive (nongated) and some are active (gated)
 Permeability = extent to which a channel permits ions to cross the membrane
 The neuronal membrane is selectively permeable
 It is more permeable to K+ than to Na+ because there are more nongated K+ channels than the
nongated Na+ channels
 Active transporters can move ions across the membrane
 ATP provides a form of fuel that neurons uses to operate these small transmembrane pumps
 Every molecule of ATP can provide enough energy to move 2 K+ ions inside for every 3 Na+
extruded
 Over time, pumping changes internal to external neuronal concentrations of Na+ and K+ and
creates ionic concentration gradients
 In the resting state, a higher concentration of Na+ exists outside the neuron and a higher
concentration of K+ exists inside
 Pumps establish concentration gradients such that there is more Na+ outside and more K+
inside
o This creates a force of unequal distribution of ions that wants to push Na+ from an area
of high concentration to an area of low concentration
o Since the membrane is more permeable to K+ then to Na+, the force of concentration
gradient pushes some K+ out causing an electrical gradient to develop
o Electrical gradient = K+ ions carry one unit of positive charge out of the neuron as they
move across the membrane, the environment outside the neuron becomes more
positive than the inside
 As the K+ leaves the cell, it gets harder for the K+ to leave because of the negative environment
inside the neuron (positive charge attracts the negative charge)
 Electrochemical gradient = When the force of the concentration gradient pushing the K+ out
through the nongated K+ channels is equal to the force of the electrical gradient acting to keep
the K+ in
 Difference of concentrations of ions across the selectively permeable membrane of the neuron
leads to the resting membrane potential
 Nernst equation proves for the calculation of that potential when one ionic species is involved  Equilibrium potential = membrane potential at which a given ion has no net flux across the
membrane; that is, as many of the ion move outward as inward
 In neurons, the resting membrane potential is equal to the equilibrium potential of K+ (-75 mV)
Electrical Conduction in Neurons
 Neurons have 2 important properties
o They are volume conductors – allow currents to flow through them and across their
membrane
o They generate a variety of electrical currents called receptor potential, synaptic
potentials and action potentials
 To record the transmembrane difference in potential, you need a small recording electrode
inside a neuron and one outside ; the difference in potential between these 2 electrodes is the
value of the membrane potential
 The action of stimulating electrode can approximate the role of the synaptic potentials in a
neuron
 Neurons are essentially sacks of electrically conductive fluid (cytoplasm) bounded by an
electrical insulator (the cell membrane) making them excellent conductors
 Neurons and their environment can be broken down into conductors (cytoplasm and ECF) and
insulators (membranes)
 Membranes have high but variable resistance and the ability to store charge (capacitance)
 Active VS. Passive currents
o When synapse is activated, active electrical currents are generated across the cell
membrane near the synapse
o These currents generate synaptic potentials
 Current flows across postsynaptic membrane in localized region causing a
passively conducted current conducted throughout the neuron (electrotonic
conduction)
 Passive current can be depolarizations (EPSP) which make inside cell more
positive and more likely to generate an action potential; or they may be
hyperpolarizations (IPSP) which make inside cell less positive and less likely to
generate an action potential
o Passive currents conducted via volume conductance through the cytoplasm and pass
through the dendrites and soma of the postsynaptic neuron
o If the passive currents sufficiently depolarize the neuron, they can trigger action
potentials in the spike-triggering zone at the axon hillock of the neuron
o Action potential is an active process because it involves the changes in membrane
conductances by opening and closing ion channels
o These passive currents can depolarize nearby patches of the axon and cause them to
generate new action potential, allowing the process to continue down and release the
neurotransmitter
 Movement of ions to the inside of the neuron is accompanied by the return of currents to the
outside of the neuron, forming a complete circuit
 The distance of current flow is a function of 3 main properties of the neuron
o Amplitude of the original current o Resistance and capacitance of the neuronal membrane
o Conductivity of the intracellular and extracellular fluid
 Placing an electrode in the axon of the neuron, the current is strongest near the electrode and
the passive electronic currents that flow down the axon decrease with the distance away from
the source
 Assuming the resistivity does not change, the change in membrane voltage is due to the
diminished amplitude of the current at more distant loci and therefore refer to the electrotonic
conduction as “decremental conduction”
 The distance from the source that electrotonic currents can still be effective for communication
depends in part on the size of the original current. (greater the current, further it will conduct)
 Passive electrotonic conduction is not a sufficient signal since it diminishes with distance so it is
not appropriate for long-distance communication but it can work well for short distances
 Under normal physiological conditions, the amplitude of the current is determined by the
physiological factors such as the intensity of a physical stimulus at a receptor, or the strength
and the number of synaptic inputs onto the neuron
 As the membrane resistivity increases, more current will be shunted down the axon and less
will leak out
 Conductivity affects how far through a neuron a current will flow
o IC and EC have high conductivities because they are generally good conductors of
electrical currents
o The resistivity of dendrites, cell bodies, and axons changes as the function of their size
o If axon is large, the current flow is greater
o High amplitude receptor or synaptic currents, high membrane resistance, and low-
resistance intracellular pathways enhance the electrotonic conduction of currents by
permitting them to affect the neuronal membrane at loci more distant from their site of
generation
 Electrotonic conduction is good for short distance communication but fails for long distance
 Long distance communication requires active or regenerative electrical signals called action
potentials
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